First Detection of [C I] ${}^3$P${}_1$-${}^3$P${}_0$ Emission from a Protoplanetary Disk

We performed single point [C I] ${}^3$P${}_1$-${}^3$P${}_0$ and CO J=4-3 observations toward three T Tauri stars, DM Tau, LkCa 15, and TW Hya, using the Atacama Large Millimeter/submillimeter Array (ALMA) Band 8 qualification model receiver installed on the Atacama Submillimeter Telescope Experiment (ASTE). Two protostars in the Taurus L1551 region, L1551 IRS 5 and HL Tau, were also observed. We successfully detected [C I] emission from the protoplanetary disk around DM Tau as well as the protostellar targets. The spectral profile of the [C I] emission from the protoplanetary disk is marginally single-peaked, suggesting that atomic carbon © extends toward the outermost disk. The detected [C I] emission is optically thin and the column densities of C are estimated to be <~10${}^{16}$ cm${}^{-2}$ and ~10${}^{17}$ cm${}^{-2}$ for the T Tauri star targets and the protostars, respectively. We found a clear difference in the total mass ratio of C to dust, $M$©/$M$(dust), between the T Tauri stars and protostellar targets; the $M$©/$M$(dust) ratio of the T Tauri stars is one order of magnitude smaller than that of the protostars. The decrease of the estimated $M$©/$M$(dust) ratios for the disk sources is consistent with a theoretical prediction that the atomic C can survive only in the near surface layer of the disk and C${}^{+}$/C/CO transition occurs deeper into the disk midplane.